1. Field of the Invention
The present invention relates to a digital receiver, and more particularly, to a method and apparatus for estimating an SFO (Sampling Frequency Offset) applicable to the digital receiver, and an apparatus for compensating for a sampling frequency using the estimated SFO.
2. Discussion of the Related Art
Generally, a DVB-T (Digital Video Broadcasting-Terrestrial) system acting as the European transmission standard for a terrestrial digital TV has generally selected an OFDM (Orthogonal Frequency Division Multiplexing) transmission scheme. It is well known in the art that the OFDM transmission scheme has very strong resistance to channel distortion caused by multiple paths (also called a multi-path) in a wireless broadband broadcast system.
On the other hand, the OFDM transmission scheme has very weak resistance to synchronization. Therefore, if accurate synchronization is not established between a transmitter and a receiver, distortion of a reception signal occurs. In order to solve the above-mentioned problem, many developers have conducted intensive research into the improved OFDM transmission scheme.
Particularly, if the receiver does not correctly perform sampling synchronization, an ISI (inter Symbol Interference) and a constellation rotation may occur in a reception signal, such that the receiver cannot demodulate the reception signal.
In order to solve the above-mentioned problem, there has been newly proposed a method for estimating an SFO (Sampling Frequency Offset) using a CP (Continual Pilot) shown in FIG. 1.
FIG. 1 shows general CP positions. As shown in FIG. 1, 45 pilots are employed during a 2 k mode, and 177 pilots are employed during an 8 k mode.
For example, in the case of the 2 k mode, a total of 1705 data subcarriers are present in one OFDM symbol interval. A pilot is located at each of subcarrier positions, for example, 0-th, 48-th, and 54-th subcarrier positions, etc. In this case, the pilot is positioned at the same subcarrier positions as the above subcarrier positions in the next OFDM symbol, such that the pilot will be referred to as a Continual Pilot (CP).
A method for calculating the SFO using the above-mentioned CP information shown in FIG. 1 is shown in FIG. 2.
FIG. 2 is a block diagram illustrating a conventional SFO estimation system.
The above-mentioned conventional SFO estimation method will hereinafter be described with reference to FIG. 2. Firstly, the SFO estimation system receives a single signal Zl,k. The Zl,k signal is indicative of a k-th subcarrier in a first OFDM symbol. For example, in the case of the 2 k mode shown in FIG. 1, the Zl,k signal is indicative of one pilot from among a plurality of pilots (i.e., 0-th, 48-th, and 53-th symbols, etc.) in the first OFDM symbol.
The Zl,k signal is converted into another signal of Zl-1,k via a delay 10. The Zl-1,k signal is converted into a conjugate root signal of Z*l-1,k via a conjugate calculator 20.
Correlation between the Zl,k signal and the Z*l-1,k signal is performed by a multiplier 30, such that the multiplier 30 outputs a phase information signal of xl,k. By the following equation 1 performed by a phase estimator 40, the phase information signal of xl,k acquires a total of 45 phase data units in the case of the 2 k mode, and acquires a total of 177 phase data units in the case of the 8 k mode.
                              tan                      -            1                          =                              Re            ⁡                          (                              x                                  l                  ,                  k                                            )                                            Im            ⁡                          (                              x                                  l                  ,                  k                                            )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
An SFO calculator 50 calculates a slope between phase data units using the phase data generated from the phase estimator 40, and calculates a mean slope, such that it calculates the SFO value.
The above-mentioned conventional SFO estimation method acquires correlation between two OFDM symbols, acquires a phase on the basis of the acquired correlation, and calculates a change rate of the acquired phase, such that it estimates the SFO value.
However, the above-mentioned SFO estimation method may incur irregular overshoots of the phase due to a deep fading phenomenon in a long ghost environment as shown in FIG. 3b, whereas it easily estimates the SFO because a phase change rate is constant in an AWGN (Additive White Gaussian Noise) environment as shown in FIG. 3a. 
The above-mentioned irregular overshoots do not affect the SFO in an acquisition mode of a sampling frequency, but it greatly affects the SFO in a tracking mode of the sampling frequency as shown in FIG. 4.
FIG. 4 shows a plurality of SFO values estimated in acquisition and tracking modes of the sampling frequency. As shown in FIG. 4, it can be recognized that a jittering range increases if overshoots occur in the tracking mode.
In this manner, if the jittering range increases due to the overshoots in the tracking mode, the increased jittering range has a negative influence upon a method for compensating for a sampling frequency by estimating a correct SFO. Furthermore, assuming that an SFO of more than 400 ppm occurs in the 2 k mode of the long ghost environment or an SFO of more than 100 ppm occurs in the 8 k mode of the same long ghost environment, the conventional SFO estimation method has difficulty in correctly estimating the above-mentioned SFO values.